To avoid confliction among communication frames such as synchronization messages for time synchronization when each of a plurality of mater devices performs time synchronization with a slave device which is a management target, a technique has been known that sets different times for different master devices to transmit the communication frame for time synchronization.
However, because different times are set for different master devices to transmit the communication frame for time synchronization in the conventional technique, such a conventional technique is not compliant with a standard under which time synchronization timing should be set at a given time, e.g., the time synchronization should be performed at a predetermined time. Under such a standard, it is difficult to avoid the occurrence of confliction due to the time synchronization.